Obesity is one of the most popular and growing diseases in America. Popular approaches to weight control such as exercise plans or low carbohydrate diets have failed to halt the American obesity epidemic. Obesity occurs when there is a persistent long-term excess in energy intake over energy expenditure, and will only be resolved when the individual makes behavioral changes that correct this energy imbalance. One reason that such behavioral changes fail to occur is that it is currently very difficult to monitor the effects of behavioral changes on the various components of energy balance under typical daily life conditions. Hence we propose the development of an implantable microsensor array that could be used to conveniently monitor biomarkers relevant to carbohydrate and fat utilization without interfering with daily life activities. Fats and carbohydrates are the body's two major sources of fuel, and their relative rates of utilization depend on factors such as age, diet, exercise intensity, and fitness level. The proposed sensor array could be used to continuously monitor systemic levels of glucose, pH, and pCO2/bicarbonate in the body both at rest and during exercise. We hypothesize that such a sensor will be sensitive enough to detect substantial increases (or decreases) in dietary fat consumption both on short term and also long term basis. In the proposed work we will develop, fabricate, and test an implantable sensor platform based on smart hydrogels and micromachined pressure sensors that would continuously measure glucose, pH, and CO2 with high long term stability and biocompatibility. The proposed equilibrium sensor array will consist of four hydrogels, in independent microcavities, that respond by swelling proportionally to the concentrations of glucose, CO2 and to the pH, the fourth hydrogel acts as a reference. The tendency of hydrogels to increase in volume in confined wells is captured by the pressure transducers. This project will be accomplished in three specific aims. The first one focuses on development and long term testing of hydrogels. Second specific aim focuses on developing the SiC platform and the third specific aim on biocompatibility coating tests and long term in vitro tests. This project will be conducted very diligently by the team of experts in the fields necessary to accomplish the goal. This project is designed to develop an implantable microsensor to continuously monitor the body's metabolism for a long term. A person may be implanted with such a sensor through a minor out patient surgery and it will not hinder the daily activities in anyway. Such a sensor would cost $30 to $50 and greatly benefit people fighting obesity and trying to restore energy balance.